fix redmint #4544
fix redmint #4544

By default there is no libvirt module on controllers node, we
should catch the error. Services on controllers don't really
use this host module, so we can just ignore the ImportError.

Signed-off-by: apporc <eace494683e27f0eb70140bd4063efbda0256f8e@gmail.com>

eayunstack/nova,eayunstack/nova